Effect of Desulfovibrio Desulfuricans on Zn and Zn 17 Al 3 Alloy Fabricated Ductile Iron Suman Dutta 1 Research Scholar, Dept. of Chemistry, Jadavpur University, Kolkata-700032, W.B., India Swapan Kumar Bhattacharya 2* Professor, Dept. of Chemistry, Jadavpur University, Kolkata-700032, W.B., India Pritish Majumdar 3 Emeritus Scientist, Dept. of Metallurgical Eng. and Material Science, Jadavpur University, Kolkata-700032, W.B., India Bidhan Chandra Ray 4 Retired Professor, Dept. of Chemistry, Jadavpur University, Kolkata-700032, W.B., India Abstract - Microbial influenced corrosion of Zn (422 gm/m2) and Zn17Al3 alloy (436 gm/m2) coated ductile iron were investigated in presence of sulphate reducing bacteria desulfovibrio desulfuricans (DD) in aqueous solution using mass loss and electrochemical measurements. The specimens were exposed in DD medium for different number of days ranging from 2 to 80 to investigate the change in corrosion rates with time. The results suggest that the open circuit potential shifts to more negative values with increased exposure time and high corrosion current is obtained in presence of bacteria in comparison to abiotic medium for both samples. Electrochemical and mass loss results demonstrate that DD accelerated pitting propagation damage due to increase in both cathodic and anodic reaction rates. Presence of DD leads to formation of a dense and thick layer of oxide and hydroxide of zinc and iron, suggest propagation of the corrosion attack beneath the oxide layer. Keywords: Zn, Zn 17 Al 3 alloy coating; Microbial corrosion; desulfovibrio desulfuricans; Pitting corrosion; Potentiodynamic polarization. I. INTRODUCTION Corrosion has been recognized as one of the most dominant forms of the detoriation process and identified as the major cause of the loss of coating from pipelines. Corrosion may attack the pipelines either internally, or externally, or both. External corrosion is a major factor contributing to the detoriation of buried pipelines by weakening the pipe wall, which increases the risk of failure. Even though buried pipelines are protected with coatings and cathodic protection, the pipelines are still vulnerable to various types of corrosion mechanisms. There are many types of bacteria that can lead to corrosion initiation or increase the corrosion rate such as sulphate reducing bacteria (SRB), acid producing bacteria, iron oxidizing bacteria, nitrate reducing bacteria etc. Among them SRB is regarded as one of the most troublesome groups of bacteria influencing the microbial influenced corrosion (MIC). The mechanism of action of SRB is usually attributed to the chemical corrosiveness of H 2 S, facilitated H + reduction at deposited FeS and biological consumption of chemically formed H 2 . MIC is one of the possible threats to buried pipelines because corrosive bacteria grow well in muddy soil environment. Reviews of bacterial corrosion were published by Postgate [1] and Booth [2]. Booth and his colleagues had made impressive progress in establishing that cathodic depolarization is a major mechanism of anaerobic corrosion by SRB, correlating corrosion rates and polarization curves [3] with the hydrogenase contents of the strain and species used. Anodic processes may influence the corrosion rate in certain strains, however. The biological corrosion of coated ductile iron has received increasing consideration in the last few decades [4-12]. Zn and Zn-Al coated ductile iron have been used increasingly for pipes for supply of drinking water, drainage, cooling towers system etc. due to its corrosion resistance, mechanical workability and resistance to bio-fouling [3]. But Zn and Zn 17 Al 3 alloy coatings are susceptible to localized damage in presence of corrosive electrolytes and different corrosive bacteria. Zn and Zn 17 Al 3 alloy coatings protect iron against corrosion by the two following effects: a barrier effect due to the continuity of the coating that separates the iron from the corrosive environment and a galvanic protection because zinc and aluminium act as a sacrificial anode to protect the underlying iron [1, 2]. Usually, a thickness of 55 μm (defined by European standard NBN EN 10240 as 396 g/m 2 ) is advised for good protection of steel against generalized corrosion in fresh water. However, a coating in which the zeta phase is absent or too thick and presents a columnar morphology [13- 16], does not protect iron from generalized corrosion. To be efficient, the outer η-layer must represent at least 45% of the thickness of the whole coating [13]. Extrapolation of Tafel lines is one of the most popular DC techniques for estimation of corrosion rate. The lines for charge transfer controlled reactions give the corrosion current density, i corr at the corrosion potential, E corr . Two thumbs rules should be applied when using Tafel extrapolation. For an accurate extrapolation, at least one of the branches of the polarization curve should exhibit linear of the semi logarithmic scale over at one decade of current density. In addition, the extrapolation should start at least 50-100 mV away from E corr . These two rules improve the accuracy of manual extrapolations [17] With respect to the determination of corrosion rate, the most accurate and precise method is probably that of mass loss. This is because the experimentation is easy to replicate and, although long exposure times may be involved, the relatively International Journal of Engineering Research & Technology (IJERT) ISSN: 2278-0181 www.ijert.org IJERTV4IS051319 (This work is licensed under a Creative Commons Attribution 4.0 International License.) Vol. 4 Issue 05, May-2015 1473